In modern image sensors, especially in mobile applications, there is a requirement to reduce the physical size of the complete system. At the same time, resolution and image quality must be maintained or improved.
Typically, an image sensor comprises a main imaging lens which focuses rays of light onto a pixel array on a substrate. The intensity of the rays of light impinging on each pixel is read out to memory, and an image is generated from the values of each pixel. In addition, to enable color imaging, a color filter is placed above each pixel.
To maintain sensitivity and hence image quality, image sensors employ micro-lenses above each pixel to concentrate the incident light on the sensitive part of each pixel. However, to enable the image sensor size to be reduced, the focal length of the main imaging lens also needs to be reduced. Consequently, for the same physical size of the pixel array, the light from the main imaging lens has an increased angle of incidence at the periphery of the pixel array.
When the ray angles are increased this results in a loss of sensitivity, and hence relative illumination across the pixel field, and also increased color cross-talk between pixels. This degrades signal-to-noise ratio and color purity. Color cross-talk is primarily caused by an angled ray passing through a given color filter but arriving at the sensitive part of a neighboring pixel. As the vertical distance from the pixel photodiodes to the color filter elements is set, increasing ray angles accentuates this problem. There is also a contribution to the color cross-talk from reflected and scattered rays from the metal layers of the pixel. Again, the color cross-talk contribution from reflected and scattered rays is increased as the ray angles increase.
U.S. Pat. No. 5,610,390 describes a method to position micro-lenses according to a function of an imaging lens exit pupil and pixel position. U.S. Pat. No. 6,008,511 describes a method to position micro-lenses according to a function which incorporates the particular color of the pixel in question. The prior art systems recognize that offsetting the micro-lens in relation to the pixel can compensate for the increased angle of incidence of rays of light impinging on the pixel array.
However, when the angle of incidence of rays of light are large the micro-lens may be required to be moved a relatively large distance. This can result in the micro-lens being placed above a metal routing of the pixel rather than the light sensitive region. By placing the micro-lens above the metal routing, the micro-lens light amplification effect is reduced as some of the focused rays are reflected off the metal. In addition, the metal routing is naturally opaque, and due to the positioning of the micro-lens, may prevent the rays from reaching the light sensitive region of the pixel.